Polyester film is used in various applications due to its outstanding properties but, on account of the molecular structure of polyester, it has poor release properties, and in order to confer such properties there is generally used the technique of coating the surface with a release component. However, this has disadvantages in that, with the surface being subjected to deformation as a result of processing, the properties deteriorate, and for reasons such as the strength of the coating layer itself being inadequate, performance falls in the case of repeated use. Furthermore, there is also the problem that because of poor adhesive strength between the release layer and the polyester layer, following use in an aqueous environment or following a retort or boiling treatment, there is a marked reduction in performance. In the latter case, there has been proposed overcoming the problem by providing a primer layer or reacting two components in the coating layer with a catalyst, etc, but not only are these methods unsuitable when used in food applications, there is also the problem of a lowering of productivity and it is difficult to stably satisfy the required properties under diverse usage conditions as described above. Now, containers such as metal cans are very popular throughout the world and are essential to our way of life and, hitherto, for the purposes of preventing corrosion, it has been common to apply to the inner and outer faces of the metal can a coating material formed by dissolving or dispersing an epoxy, phenolic or other such thermosetting resin in a solvent, so as to coat the metal surface. However, this kind of thermosetting resin coating method requires a long time for the applied material to dry, so that productivity is reduced, and there is also the problem that such a method is undesirable from the point of view of environmental contamination due to the large amount of organic solvent employed.
As a method for resolving these properties, there is the method of laminating a film to the metal can material, namely steel or aluminium sheet or metal sheet obtained by subjecting such metal sheet to a surface treatment like plating. When a metal can is produced by subjecting the film-laminated metal sheet to deep-drawing or ironing, the following properties are demanded of the film.
(1) It should be outstanding in its lamination to the metal sheet.
(2) It should be outstanding in its adhesion to the metal sheet.
(3) It should be outstanding in its processability and no defects such as pin holes should be produced following fabrication.
(4) The polyester film should not separate away, or show cracking or pin hole generation when the metal can is subject to impact.
(5) There should be no adsorption of aroma components from the can contents by the film, nor will the flavour of the contents be harmed by materials dissolved out of the film (referred to below as the taste characteristics).
(6) The contents will not stick to the can walls or to the can bottom, and it will be possible to remove the contents readily (below this is referred to as the non-stick property).
Many proposals have been made hitherto in order to meet these requirements. For example, in U.S. Pat. No. 5,240,779 and U.S. Pat. No. 5,384,354, there are disclosed copolyester films of specified density and planar orientation coefficient, or polyester films based on a combination of specified polyester components. However, these proposals do not altogether wholly satisfy the aforesaid diverse property requirements and, in particular, in applications where non-stick properties are further demanded along with outstanding lamination, impact resistance and heat resistance, it is difficult to simultaneously provide such characteristics.